[aws-amplify] Creating an Amplify app fails to run build, manual intervention needed.

Creating an Amplify app fails to run build, manual intervention needed. Running it manually works.

### Reproduction Steps

```js
const amplifyApp = new amplify.App(this, 'XXX', {
sourceCodeProvider: new amplify.GitHubSourceCodeProvider({
owner: 'XXX',
repository: 'XXX',
oauthToken: cdk.SecretValue.plainText(process.env.PAT as string)
}),
});
const master = amplifyApp.addBranch('master');
amplifyApp.addCustomRule(amplify.CustomRule.SINGLE_PAGE_APPLICATION_REDIRECT);
const domain = amplifyApp.addDomain('XXX');
domain.mapRoot(master);
domain.mapSubDomain(master, 'www');
```

`amplify.yml` is repo

### What did you expect to happen?

Amplify app goes live.

### What actually happened?

On the Amplify console I can see the first build never being triggered:


### Environment

- **CLI Version :*6.14.6*
- **Framework Version:**
- **Node.js Version:*v14.6.0*
- **OS :*Ubuntu*
- **Language (Version):*TS*

### Other

---

This is :bug: Bug Report

Start by reproducing the TypeScript example with the specified AWS CDK CLI, Node.js, and Ubuntu environment, then inspect the Amplify app, branch, and domain provisioning path. Compare the initial console state with a manually triggered build. Done means creating the app triggers its first build without manual intervention and the Amplify app goes live.

Written by the indexing model from the issue text.